This sounds like a classic case of a blocked pilot jet , the concentric carbs are known for it and can be a right pain in the a%$e to clear because of the pressed in jet. I have even resorted to drilling out the brass jet from the body and fitting a removable jet in the original position which is also used for 2 strokes. the carbs on my commando have been like this for a couple of years now and work fine.

Thanks everyone for their input so far. I think that for the sake of originality and long term reliability I will go for a new Monobloc. According to the FW Neill book http://archives.jampot.dk/book/Workshop ... .NEILL.pdf I need a 389/18 with a 390 main jet, slide 4, pilot 20 and needle jet .106. Does that agree with what others are using? Does the price for carbs from the spares scheme include jetting?

So, I've ordered a new Monobloc from the spares scheme - but it's on back order. Apparently they can't get hold of a #4 slide. An order was put in to Amal in November, but they just say they will be available 'soon'. I'll have to see if I can coax some enthusiasm out of the Concentric that's on there.

Thanks to everyone for their advice, and to Leswaller and Spriddler for their suggestion for locating a #4 slide. But I don't need to bother, as my new Monobloc arrived this morning, all jetted up and raring to go. And a beautiful thing it is too. Can't wait to fit it.

MalcW wrote:Thanks to everyone for their advice, and to Leswaller and Spriddler for their suggestion for locating a #4 slide. But I don't need to bother, as my new Monobloc arrived this morning, all jetted up and raring to go. And a beautiful thing it is too. Can't wait to fit it.

Malc

That’s how I felt when it arrived. Made such a difference, you will love it, bike ticks over a treat now, like a Singer.

So, I've started fitting the Monobloc to the Model 31. Firstly, I had to move an ugly cartridge oil filter that someone previous had fitted, as it fouled the Monobloc's float chamber but that was a job I was going to do anyway. One thing though is that the clearance between the carb's cable adjusters and the frame tube is very tight, especially for the choke adjuster. Is this normal?